Hybrid electric vehicles employ an electric drive system that has lower energy costs and emits fewer pollutants than a conventional internal combustion engine (ICE) drive system. Various configurations of hybrid electric vehicles have been developed. In a first configuration, an operator can choose between electric operation and ICE operation. In a series hybrid electric vehicle (SHEV) configuration, an engine, typically an ICE, is connected to an electric motor referred to as a generator. The generator, in turn, provides electricity to a battery and another motor referred to as a traction motor. In the SHEV, the traction motor is the sole source of wheel torque. There is no mechanical connection between the engine and the drive wheels. In a further configuration, a parallel hybrid electric vehicle (PHEV), an engine and an electric motor cooperate to provide the wheel torque to drive the vehicle. In addition, in a PHEV configuration, the motor can be used as a generator to charge the battery from the power produced by the ICE. A further configuration, a parallel/series hybrid electric vehicle (PSHEV), has characteristics of both the SHEV and the PHEV.
Electric propulsion in an HEV can be performed by an electric drive system that can include a number of components, typically at least including a power conversion circuit and a motor. In this arrangement, the power conversion circuit can controllably transfer power from a power source to the motor to drive a load. A typical power conversion circuit can comprise a power source, such as a high voltage battery, and an inverter circuit that can provide a three-phase current to an electric machine. As discussed above, at times a hybrid vehicle can operate in an electric drive mode, propelled solely by an electric motor, and at other times an ICE can cooperate to drive the vehicle. In the past, a planetary gear arrangement has been employed in hybrid vehicles to turn over an internal combustion engine when it is to assist in driving the vehicle. However, in some hybrid designs, the planetary gear arrangement can cause an unacceptable torque disturbance manifested as a vehicle shuddering effect.
As an alternative to a planetary gear arrangement in a hybrid vehicle, a starter motor can be employed. For example, an integrated starter generator (ISG) can be used to both drive a vehicle and start an ICE. However, in many cases the ISG current draw necessary to start the engine is too high for the drive system DC-DC converter, causing it to sag in terms of voltage output. In a further configuration, a separate low voltage starter motor, rather than an ISG, can be used to start the ICE. In this type of configuration, the DC-DC converter can be coupled to the starter motor by a low voltage bus. A low voltage bus can also link a vehicle's low voltage battery and a low voltage load, such as lamps, exterior lights, a radio, and other vehicle accessories to the DC-DC converter. Thus a high voltage battery used in a conversion circuit to power an electric motor can be used to power low voltage vehicle accessories and a low voltage starter motor for an ICE through a DC-DC converter.
Unfortunately, this configuration suffers drawbacks as well. A conventional starter motor can draw a relatively large current that can cause a significant voltage drop, perhaps down to 6-8V, elsewhere along the voltage bus. In general, when a driver starts a conventional ICE vehicle, few, if any, accessories are turned on. Those accessories that are powered on may experience a brief flickering or fading that ends with the cranking of the engine, no more to occur during the drive. Because they occur temporarily only at the beginning of a vehicle's operation, the effects of a large starter motor current draw is not a significant issue for ICE vehicles. However, in a hybrid vehicle, an engine can be turned on and off frequently during a single driving episode. Repeated flickering lights or fading radio volume due to insufficient voltage on a low voltage bus can become an annoying, unacceptable nuisance to an HEV driver.
In the past, various solutions have been proposed to address the issue of voltage drops due to large starter current demand. For example, U.S. Patent publication 20090107443A1 to Sarbacker et al. discloses a controller for turning off the engine when the vehicle is idle, a motor/generator for starting the engine, an inverter for converting a DC auxiliary voltage from a battery into an AC voltage for powering the motor/generator, and a device for isolating a DC voltage from the DC auxiliary voltage to prevent voltage sag in a vehicle system during engine starting. The device includes a transformer, a rectifier/regulator, and an isolator. From a single energy storage device, such as a low voltage battery, a DC voltage can be isolated from a DC auxiliary voltage and provided to an auxiliary system comprising components that are susceptible to a voltage sag while an engine is being cranked. A method includes detecting a commanded engine start, comparing a measured auxiliary voltage to a threshold, isolating a predetermined DC voltage from a DC auxiliary voltage when the measured auxiliary voltage is less than the threshold, and powering the auxiliary vehicle system using the isolated DC voltage. The energy storage device can be charged by the motor/generator. The Sarbacker solution depends on a single battery providing sufficient voltage for both a starter motor and an auxiliary system. Sarbacker teaches the addition of a transformer, a regulator and an isolator, and relies on a comparison of voltages and a division of vehicle accessories between those that are subject to voltage sags and those that aren't.
U.S. Publication 20090243387 to Cohen et al. discloses a dual battery electrical system having a primary and a secondary load, and is switchable between an ON state in which the engine is running, an OFF state in which the engine is not running, a START state in which the primary load requires power to start the engine, and a PAUSE state in which the engine is not running. A first battery powers the primary load, requiring power to start an engine, and a second battery powers the secondary load, not requiring power to start an engine. A battery switch is closable to connect the first battery to the second battery such that both batteries can provide power to both loads. The switch is open or closed dependent on the vehicle condition and operational state. Cohen teaches disconnecting the two batteries so that the primary battery has sufficient charge to start an engine. In addition, the health of the primary battery can be checked prior to connecting or disconnecting the two batteries. As the Cohen invention is directed toward having sufficient power in the primary battery to start an engine, the issue of voltage sag in the secondary load is not emphasized.